In the oil and gas production industry, pieces of equipment are often required to be suspended in a well. For example, a pump assembly (comprising a down hole pump and a motor) may be needed in a well to assist in lifting fluids from the bottom of the well to the surface. The main components of a typical arrangement are shown in FIG. 1. According to conventional practise, the pump assembly (1) is attached to the lower end of a jointed tubing string (2) so that the jointed tubing holds the pump assembly in position in the well (3). The jointed tubing string is normally the production tubing string and comprises a series of rigid tubing sections joined together. To supply power to the pump assembly, an electrical cable (4) is run along the outside of the jointed production tubing string and clamped thereto using cast iron clamps or tensioned steel bands (5) so that the production tubing string supports the weight of the electrical cable. The lower end of the electrical cable is connected to the pump assembly.
The production tubing string (2) is suspended from a well head (6) by way of a connector (8) so that fluids can flow from the bottom of the well, up the production tubing and through a channel (9) in the well head. The well head is the assembly at the top of the well casing that seals the top of the annulus between the well casing and the production tubing. The connector (8) has a conical outer profile and a threaded inner bore. The top of the production tubing string is also provided with threads so that it can be screwed into the connector (8). The connector sits in a conical seat so as to hang the production tubing string in the well casing.
A Christmas tree (not shown in FIG. 1) connected to the top of the well head includes an arrangement of valves for directing fluids to/from the well and for sealing off the well when desired.
The electrical cable (4) is separated from the jointed production tubing string (2) just below the well head (6) and run through a separate fluid to air connector (7). The fluid to air connector allows electrical cables to pass from the well to the external environment whilst preventing the flow of fluids between the well and the external environment.
Several types of fluid to air connector have been used in the industry. For example, the fluid to air connector can be in the form of a rubber seal which can be arranged annularly around the electrical cable and which can be compressed so as to seal around the cable.
However, for reliability, the fluid to air connector (7) is typically a penetrator. Penetrators are well known in the oil and gas production industry and often comprise a steel cylinder through which the electrical cable runs, the space between the inner wall of the cylinder and the cable being sealed with epoxy resin, elastomer packings or similar. The steel cylinder has a smooth outer surface around which a seal can be formed easily, or alternatively recesses for o-rings which seal in a smooth bore provided in the well head. Such penetrators are widely available, for example from Remote Marine Systems, ITT Cannon, Teledyne D. G. O'Brien, Quick Connectors Inc, or Diamould.
As failure of the fluid to air connector (7) would compromise the ability of the well head to contain well pressure, it is a safety critical component. Consequently, in typical arrangements, penetrators are used which have been manufactured with a portion of electrical cable extending there-through. The seal is then tested in the factory. The lower end of the electrical cable extending through the penetrator is then spliced onto the upper end of the pump assembly's electrical cable in situ. In this way, a reliable seal through the fluid to air connector is obtained.
This conventional arrangement of a fluid to air connector (7) which is separate from the channel (9) passing through the well head for the flow of well fluids is widely used as the presence of the electrical power cable does not interfere with the operation of the Christmas tree valves, so the valves can be readily closed to seal off the well if necessary.
Conventional methods of installing pump assemblies on jointed production tubing in wells requires the use of expensive workover rigs to remove and re-install the production tubing when maintenance or repair of the pump assembly is required. This process is also time consuming.
U.S. Pat. No. 5,670,747 discloses an apparatus for installing an electric submersible pump within an oil well. In U.S. Pat. No. 5,670,747, production tubing extends from the wellhead to the electric submersible pump, i.e. the electric submersible pump is suspended from the wellhead by the production tubing. In use, pumped fluids travel from the pump to the wellhead within the production tubing. One or more packers are located at a plurality of depths within the well to seal the annulus between the production tubing and the well casing. Electrical power is transmitted to the electric submersible pump via a series comprising a wellhead penetrator, an electric submersible pump (ESP) cable, a packer penetrator and a motor lead extension. The ESP cable and the motor lead extension are attached to the outside of the production tubing. The cable and the motor lead extension comprises a substantially rigid outer metal jacket whose principal function is to provide abrasion resistance and some mechanical protection to the electrical core of the cable. However, the substantially rigid outer metal jacket is not intended to support an applied axial load, nor is it capable of so doing. Indeed, rather than the cable supporting the weight of the pump assembly, the pump assembly and the cable along its entire length are supported by the production tubing.
Alternative methods of installing pump assemblies have been proposed to avoid the expense and time involved with installing pump assemblies on jointed tubing. Such methods include installing the pump assembly on the end of spoolable tubing or on the end of an electro-mechanical cable. Where spoolable tubing is used, the electrical cable to power the pump assembly is either run along and secured to the outside of the spoolable tubing in the same way as with jointed tubing, or is supported within the spoolable tubing, as found with a tubing-encapsulated power cable.
These methods require a means of terminating and supporting the spoolable tubing or electro-mechanical cable at the well head, and also a means of routing the electrical cable through the well head.
General methods of terminating and supporting spoolable tubing and mechanical cable are well known, including (but not limited to) the use of wedge shaped slips, conical or seated tubing hangers, concentric cone rope sockets, swaged ferrules, spelter sockets, wedge sockets, collets etc.
As regards routing the electrical conductors through the well head, where an electrical cable is provided on the outside of spoolable tubing, the electrical cable can be routed in the same way as with jointed tubing. However, this method is rarely used because of its limited advantages over conventional jointed tubing. Where a tubing-encapsulated power cable or an electro-mechanical cable is used, two methods are known to have been used to date.
Firstly, the conical profile provided within the well head for suspending the production tubing string has been used with an adaptor to suspend the tubing-encapsulated power cable or electro-mechanical cable in the well. The electrical conductors are then routed through the centre of the well head and Christmas tree, exiting via a fluid to air connector above the Christmas tree valves. Alternatively, a hanger which includes a pressure containing chamber and a penetrator there-through is bolted to the top of the Christmas tree and the tubing-encapsulated power cable or electro-mechanical cable is suspended there from and thereby extends through the Christmas tree and well head. The electrical conductors extend through the penetrator. In all cases, the electrical conductor and in some cases the tubing encapsulation is routed through the Christmas tree valves, rendering it impossible to isolate the well directly above the well head flange using the lowest (master) valve on the Christmas tree. The ability to isolate the well for routine maintenance is severely restricted. Further, the entire Christmas tree above the well head flange is subject to well pressure, which increases the risk of a leak of well fluids. Further, the adaptor required to hang the tubing-encapsulated power cable or electro-mechanical cable from the conical profile provided within the well head to suspend the production tubing must be available in a wide variety of sizes/types. This is because there are a wide range of well heads so one adaptor would not be suitable for all types of well head. These adaptors are also normally bulky and heavy to accommodate the large difference in diameter between production tubing (which the conical profile is designed to fit) and tubing-encapsulated power cable or electro-mechanical cable. Accordingly, a large store of different adaptors is required to hang tubing-encapsulated power cable or electro-mechanical cable from a conical profile in a conventional well head.
The invention has been made with these points in mind.